Anything I can do with a +Res -Def Klein?

Standard Death Blow/Brave Bow+ Klein with his default kit is the way to go. You can replace his B skill though as he lacks the bulk and speed to utilize Quick Riposte. He'll be a really solid archer for you on Arena Assault/Chain Challenges for little investment.

You can do with Klein what he is really good for. Fodder him to someone else.

+res -def isn’t horrible, attack isn’t touched so he can still do what he’s meant to do. If you like him then I see no reason not to build him, this is coming from me and I wound up building and upgrading my 4 Star +hp -atk Leon and he still fights very well. Basically if you want to use the unit, go on ahead, those IVS aren’t that helpful but they don’t hurt him much either so I see no problem

If you have any spare tome breakers lying around he'd be a good bait unit for arena assault (with Fortify/Spur/Rally Res support), since he'd obliterate any mage on the next player phase.

Having said that, if this is just a 4* Klein, I would 100% use him for DB3 fodder. The skill's too valuable to bother working with sub-par 4* IVs.